Slurry processes for the homopolymerization and copolymerization of monomers, especially olefin monomers, are well known in the art. Such processes can be conducted, for example, by introducing the monomer or monomers into a stirred and/or circulated stream of resin particles, catalyst and diluent. As used herein, the term “slurry reactor” includes any reactor suitable for polymerizing monomers in a heterogeneous medium including diluent (solution phase) and polymer product (solid phase), such as a stirred slurry reactor or a slurry loop reactor. As used herein, the term “slurry reactor system” includes the slurry reactor and associated recycle systems and product drying and recovery systems.
The properties of the polymer produced in a slurry reactor system are affected by a variety of operating parameters, such as temperatures, monomer feed rates, slurry density, and catalyst feed rates. In order to produce polymer having a desired set of properties, such as melt index and density, polymer product exiting the reactor is conventionally sampled and laboratory measurements carried out to characterize the polymer product. If it is discovered that one or more polymer product properties are outside a desired range, polymerization conditions can be adjusted, and the polymer product resampled. This periodic sampling, testing and adjusting, however, is undesirably slow, since sampling and laboratory testing of polymer product properties such as melt index, molecular weight distribution, and density is time-consuming. As a result, conventional processes can produce large quantities of “off-spec” polymer product before manual testing and control can effectively adjust the polymerization conditions. This occurs during production of a particular grade of resin as well as during the transition process between grades.
Methods have been developed to attempt to provide rapid assessment of certain polymer properties and rapid adjustment of polymerization conditions. PCT publications WO 01/09201 and WO 01/09203 disclose Raman-based methods using principal components analysis (PCA) and partial least squares (PLS) to determine concentrations of components in a slurry reactor. The concentration of a particular component, such as ethylene or hexene, is determined based on measurements of a known Raman peak corresponding to the component. U.S. Pat. No. 5,999,255 discloses a method for measuring a physical property of a polymer sample, preferably nylon, by measuring a portion of a Raman spectrum of the polymer sample, determining a value of a preselected spectral feature from the Raman spectrum, and comparing the determined value to reference values. This method relies on identification and monitoring of preselected spectral features corresponding to identified functional groups, such as NH or methyl, of the polymer.
Additional background information can be found in U.S. Pat. Nos. 6,479,597, 6,144,897 and 5,151,474; European Patent application EP 0 561 078; PCT publication WO 98/08066; and Ardell, G. G. et al., “Model Prediction for Reactor Control,” Chemical Engineering Progress, American Institute of Chemical Engineers, U.S., vol. 79, no. 6, Jun. 1, 1983, pages 77–83 (ISSN 0360-7275).
It would be desirable to have methods of determining polymer product properties such as melt index, density, molecular weight distribution, and tab swell, on-line in a slurry reactor system, without the need to preselect or identify spectral features of a polymer to monitor. It would also be desirable to have methods of controlling a slurry reactor system to maintain desired polymer product properties, based on a rapid, on-line determination of the polymer product properties.